This invention relates to deposition of protective coatings or films on various substrates such as glass, metals, and plastics. More particularly the invention is directed to adherent protective coatings on plastic surfaces such as sheets, films, and shaped articles. Coatings which are abrasion resistant and protect against radiation damage to the plastic substrate are provided by the invention disclosed herein. The term protective coating means one or more layers of deposited material which provides protection against abrasion and UV degradation and reflects IR radiation.
The technological importance of thin films has led to the development of a variety of deposition methods.
Chemical vapor deposition (CVD) produces a solid film on a substrate surface by thermal activation and surface reaction of gaseous reagents which contain the desired constituents of the film. Energy required to pyrolyze the reactants is supplied by heating the substrate. For reasonable reaction rates the substrate is heated to relatively high temperatures in the range of about 500 to 2000xc2x0 F. degrees. These temperatures preclude application of the process to heat sensitive substrate materials.
Plasma enhanced chemical vapor deposition (PECVD) supplies energy to the reactants by an electrical discharge in a gas which forms a plasma in the deposition chamber. Generally the substrate is immersed in the plasma.
Polycarbonate is often the engineering material of choice for glazing and optical applications because of its high impact strength, low density, optical clarity, and good processibility. However, the polycarbonate material is soft, lacks glass-like abrasion resistance, and is sensitive to temperatures above about 300xc2x0 F. Prior work had shown that a silicon oxide coating by plasma-enhanced chemical vapor deposition (PECVD) can improve the abrasion resistance of polycarbonate, qualifying it for glazing applications. However, the prior PECVD technology using silane and nitrous oxide as the precursors was slow and therefore uneconomical, having a typical deposition rate of only about 0.05 microns per minute. Organosilicon precursors were later used in PECVD for a plasma-generated abrasion-resistant polymer coating, but the deposition rate was not significantly improved.
A plasma arc method for preparing a clear weather stable protective coatings on polycarbonate (PC) has been developed. The coating can be essentially stoichiometric silicon dioxide or silicon oxide-based which contains small amounts of carbon and hydrogen. The term xe2x80x9csilicon oxide-basedxe2x80x9d as used herein means a material which comprises oxides of silicon and small amounts of carbon and hydrogen, organic residue from the organosilicon compounds used to form the material. The coating imparts glass-like abrasion resistance to the polycarbonate article. The coating was deposited at rates of up to about 20 microns per minute at a reduced pressure with oxygen and an organosilicon compound such as hexamethyidisiloxane (HMDSO) injected into an argon plasma generated by an arc plasma torch. In the practice of this invention the surface to be coated is positioned in the path of the active species generated by the plasma as they pass into and through the deposition or coating chamber of the apparatus.
The polycarbonate substrate surface can be pretreated with a primer interfacial layer before deposition of the abrasion resistant coating material. The term xe2x80x9csubstratexe2x80x9d, as used herein, refers to a structure such as a sheet or film which acts as the base or support for the material which forms the coating or is one of a series of coatings. Generally, the substrate, although it can be a thin film, is relatively thick compared to the thickness of the coating.
This invention comprises methods for high-rate deposition of silicon oxide-based weather-stable, abrasion-resistant, and radiation-stable protective coatings on plastics such as polycarbonate. The term xe2x80x9chigh ratexe2x80x9d deposition refers to deposit of coatings at a rate greater than about 5 microns per minute. The coatings must be optically clear and abrasion resistant and may satisfy other functional requirements such as providing infrared or ultraviolet protection, and adhesion improvement. The coating process must be operative at temperatures below the thermal damage threshold, generally the glass transition temperature, of the plastic substrate or an intermediate functional layer, such as an ultraviolet radiation absorbing layer, on which the coating or coatings are deposited. It is preferred to operate with the substrate at a temperature at least 20xc2x0 C. below the glass transition temperature, e.g. about 135xc2x0 C. for polycarbonate substrates.
A plasma deposition method for coating abrasion resistant coatings on plastic using an expanded thermal plasma of argon generated by an arc plasma torch and injecting an organosilicon precursor and oxygen into the plasma as it exits the torch to deposit a silicon oxide-based coating under reduced pressure on the target surface has been developed. Polycarbonate film (10 mil thick) and sheet (0.125xe2x80x3-thick) pre-coated with about 4 to 8 microns of a silicone hardcoat were coated with silicon oxide-based using the plasma deposition method of this invention without direct cooling of the substrate. Optically clear coatings free of microcracks were produced without thermal damage to the substrate. The silicon oxide coating greatly improved the abrasion resistance of the composite as evidenced by Taber abrasion tests. The term xe2x80x9ccompositexe2x80x9d refers to the substrate with its silicon oxide-based abrasion resistant coating and any other functional coatings which may be present.
A water-cooled arc with a 4 mm cylindrical bore was used to deposit the coatings as described herein. The arc generator consists of a copper anode separated from 3 needle cathodes of thoriated tungsten by at least one electrically isolated metal plate. With argon flowing, a dc voltage is applied to the electrodes to generate a plasma. The plasma expands through a diverging or bell-shaped nozzle-injector into a deposition chamber maintained at a reduced pressure by a vacuum pump. The stainless steel nozzle-injector has two or more shower-ring gas distributors for the injection of reactive gases into the argon plasma stream. The nozzle-injector is heated to a temperature sufficient, e.g., about 200xc2x0 C., to avoid condensation of reactive gas organosilicons such as HMDSO. The substrate is mounted on the jet axis by means of a metal stage at a working distance of about 15 to 70 centimeters from the anode. A retractable shutter can be inserted between the mounting stage and the nozzle to regulate the exposure of the substrate to the plasma. Polycarbonate and silicon hardcoated polycarbonate substrates can be prepared for deposition coating by washing with isopropyl alcohol and vacuum drying at about 80xc2x0 C. to remove volatile contaminants.
The bore or central channel of the plasma torch need not be cylindrical. The bore can be cone shaped, widening as it approaches the discharge end of the torch.